1.3.3 Networks

• Firewall- prevents unauthorised access between two networks. Can be implemented on both hardware and software.
• Data filtering- packets of data are inspected by the firewall to check which part they attempting to access.
• Proxy servers- Makes a web request on behalf of the recipient's computer, hiding the true network addresses of the source.
• Encryption- encoding a plaintext message so that it cannot be deciphered unless you have a numerical key to decrypt it.
• • If a message is intercepted, it will not be understood due to encryption.
  • if the key can be intercepted, the encryption process is rendered useless.
• Trojans- malicious software programs that hide as harmless/useful applications.
• Trojans...

  • cannot self-replicate
  • are often used to make the processing power, internet bandwidth and data of a computer can be exploited remotely
• Phishing- using email to manipulate a victim into visiting a fake website and giving away personal information.
• Buffer overflow- when data that is too big for its location gets rewritten to a different location causing malware to manipulate it.
• Malware can deliberately cause and manipulate overflow data, which may be read as malicious instruction.
• SQL injection- when a malicious user enters SQL commands via online database forms
• A worm is a standalone malware program that exploit vulnerabilities in the destination system and spreads automatically.
• Prevention of malware:

  • Up-to-date patches to the operating system and application programs reduce vulnerabilities in the system.
  • Up-to-date anti-malware/anti-virus software can prevent the spread of infection.
• Malware:

  • Damages users data 
  • Worms and viruses self-replicate
  • A virus infects (embeds itself in) other programs or data files
  • A virus needs a user to help it spread
• Packet filtering:

  • Packets of data are inspected by a firewall to check which port they are attempting to access.
  • Different network protocols use different port numbers.
  • If this traffic is to be allowed through, the port must be opened for the duration of the connection, otherwise, the firewall will automatically reject it.
• Network topology- the arrangement of the various computing devices which make up a computer network.
• Bus topology- a network arrangement where nodes are connected in a daisy chain by a single central communications channel.
• Features of a bus network

  • All nodes are connected to a single backbone cable
  • Each end of the backbone is connected to either a terminator or a computer which stops signals 'bouncing back'
  • Each node is passive
  • Data is sent in one direction at a time
  • Only one computer can transmit successfully at any time
• Advantages of a bus network:

  • Inexpensive to set up
  • Devices can easily be added
  • Works on a small network
• Disadvantages of a bus network:

  • The main cable is a point of failure
  • Limited cable length
  • Performance degrades with heavy use, owing to data “collisions”
  • Poor security
• star topology- an arrangement where a central node/hub provides a common connection point for all other nodes.
• Features of a star network

  • computers are connected to a central node. This is often a switch
• A switch sends each communication to the specific computer it is intended for.
• Advantages of a star network

  • Easily add new nodes
  • Switching off a node doesn't affect the rest of the network
  • More secure if a switch is used as data is sent only to recipient
• Disadvantages of a star network

  • If the switch stops working, the whole network fails
  • Can be expensive to set up
• Internet- A network of inter-connected networks
• World Wide Web (WWW)- A collection of resources accessed via the internet
• Uniform Resource Locator (URL)- used to specify the means of accessing a resource across a network and its location
• Structure of the Internet

  • Each continent uses backbone cables fed across the sea.
  • National Internet Service Providers (ISPs) connect directly to backbones.
  • ISPs distribute internet connection to smaller providers who provide internet access to homes and businesses.
• • The address length of IPV4 is 32 bit
  • The address length of IPV6 is 128 bit
• circuit switching- creates a direct link between two devices for the duration of the communication.
• packet switching- a method of communicating packets of data across a network.
• Data packets

  • when sending across a network, data is broken into chunks/units called data packets and are assembled again at the receiving end.
• Latency- how long it takes for data to come from one place to another.
• Building a packet

  • A data packet is a segment of data that needs to be sent
  • Packets are sent into three sections: The header, the payload and the trailer.
  • The header contains the information about the data
  • The payload is the data itself varying from 500 to 1500 bytes
  • The Trailer is the end of the packet.
• Packet size

  • packets are deliberately kept small
  • this ensures that individual packets do not take excessive time to transfer preventing other packets from moving
• Packet trailer

  • Trailer checks if data is error-free
  • Receiver checks data using Checksums or Cyclical Redundancy Checks (CRCs)
  • if the checksum doesn't match the request, data is considered corrupted and a new request will be sent.
• • Gateways are required where data is travelling from one network to another that use different protocols.
• Gateways

  • Networks using different transmission media can require a gateway
  • Header data are removed and reapplied using the correct format of the new network
  • A router and a gateway can often be combined into one integrated device
• TCP/IP protocol stack- a set of rules used in turn, to format a message so it can be sent over a network.